SAPs, namely highly water-swellable polymers, typically are prepared by solvent or solution polymerization of an aqueous mixture of monomers. Usually, one or more X-linking agents are incorporated into the monomer mixture. When the polymerization has ended, the resultant is dried and subjected to mechanical grinding to create a desired particle size distribution for the particulate SAP.
As noted above, SAPs are useful in various absorbent articles, due to the ability of the SAPs to absorb bodily liquids in a ready manner. However, SAPs also absorb water from the air, and when exposed to humid environments, tend to agglomerate together into a large mass, i.e., to cake. Consequently, the SAP particles are not free-flowing.
Problems arise when SAP particles are not free-flowing. For instance, the particles are difficult to incorporate into absorbent articles because the reduced flowability interferes with the uniform distribution of the particles within an absorbent core. Additionally, the particles tend to stick to each other and to the manufacturing equipment, such as screens, driers, and fabricating machinery. Thus, production must be periodically stopped so that the equipment can be cleaned by removing the agglomerated particles. Finally, if the particles have already absorbed some water and agglomerated, then the water-swelling capacity of the end use absorbent article will be decreased.
Various attempts have been made to reduce the caking tendencies of SAPs. One way has been by blending SAP particles with silica. For instance, U.S. Pat. No. 4,734,478 (issued Mar. 29, 1988) to Tsubakimoto, Shimomura, Irie, Masuda, Kimura, and Hatsuda, assignors to Nippon Shokubai Kagaku Kogyo, shows mixing 0.01 to 10 parts by weight of finely divided silica with 100 parts by weight of superabsorbent polymer. Also, WO 94/22940 (published Oct. 13,1994) to Staples, Wood, and Treptow, assignors to Dow Chemical, shows mixing less than 10% by weight of fumed silica with superabsorbent polymer. Additionally, U.S. Pat. No. 5,419,956 (issued May 30, 1995) to Roe, assignor to The Proctor & Gamble Company, shows an absorbent end use article that includes very small SAP particles (at least 70% by weight will pass through a U.S. Standard 50 mesh sieve which has openings of 300 .mu.m in size) that are formed by solution polymerization and that are mixed with amorphous silica. It is noted that silica occurs naturally as quartz, sand flint, and agate.
Surfactants also have been used in attempts to reduce the tendency of superabsorbent polymer particles to agglomerate, as shown for instance, in both U.S. Pat. No. 4,286,082 (issued Aug. 25, 1981) to Tsubakimoto, Shimomura, Irie, and Masuda, assignors to Nippon Shokubai Kagaku Kogyo, and U.S. Pat. No. 4,381,782 (issued May 3, 1983) to Mazurak and Fries, assignors to Kimberly-Clark. However, surfactants can also reduce the surface tension of the superabsorbent polymer, which, in turn, interferes with the ability of the polymer to absorb and to hold liquid.
Of additional interest in connection with various other compounds used in attempts to reduce the tendency of superabsorbent polymers to agglomerate are the following. U.S. Pat. No. 5,728,742 (issued Mar. 17, 1998) to Staples, Henton, Rose, and Fialkowski, assignors to Dow Chemical, shows quaternary ammonium salts. Canadian Patent No.2,180,590 (issued Jan. 8, 1997) to Engelhardt, Stuven, Daniel, and Herfert, assignors to Hoechst Aktiengefellfchaft, shows wax. Also, European Published Patent Application No. 0 705 643 A1 (published Apr. 10, 1996) to Sumlya, Koike, and Tanaka, assignors to Sanyo Chemical, shows silicone oil.
Lastly, the following are of interest in connection with general background information vis-a-vis well known procedures for the manufacture of SAPs.
The journal article "Keeping Dry with Superabsorbent Polymers", Chemtech, (September, 1994) by Buchholz, contains an excellent discussion of conventional methods for making SAPs, as well as various uses for SAPs, such as in the above-noted sanitary articles (i.e., diapers, incontinence garments, etc.), in a sealing composite between concrete blocks that make up the wall of underwater tunnels, and in tapes for water blocking in fiber optic cables and power transmission cables.
A good discussion of the methods for making SAPs can also be seen in U.S. Pat. No. 5,409,771 (issued Apr. 25, 1995) to Dahmen and Mertens, assignors to Chemische Fabrik Stockhausen GmbH. More specifically, this patent mentions that commercially available SAPs are generally X-linked polyacrylic acids or X-linked starch-acrylic-acid-graft-polymers, the carboxyl groups of which are partially neutralized with sodium hydroxide or caustic potash. Also mentioned is that the SAPs are made by two methods, namely the solvent or solution polymerization method and the inverse suspension or emulsion polymerization method.
In the solvent or solution polymerization method, an aqueous solution, for instance of partially neutralized acrylic acid and a multi-functional network X-linking agent, is converted to a gel by radical polymerization. The resultant is dried, ground, and screened to the desired particulate size.
On the other hand, in the inverse suspension or emulsion polymerization method, an aqueous solution, for instance of partially neutralized acrylic acid, is dispersed in a hydrophobic organic solvent by employing colloids or emulsifiers, and polymerization is started by radical initiators. Water is azeotropically removed from the reaction mixture after completion of polymerization, followed by filtering and drying the resultant product. Network X-linking may be accomplished by dissolving a polyfunctional X-linking agent in the monomer solution.
The disclosures of all patents and published patent applications that are mentioned are incorporated by reference.